The present invention relates to solid-state image sensors, specifically to image sensors that have an improved sensitivity and improved modulation transfer function (MTF) in the Near Infra-Red (NIR) spectral region.
The state of the art image sensors that have enhanced QE and high MTF in the NIR spectral region are typically fabricated on substrates that have low doping and thus high resistivity. The reason for this choice is to obtain a large depth of the depletion region under the sensor surface. The near IR illumination has a progressively smaller absorption as the wavelength increases and this causes photons to penetrate deeper into the substrate. The generated electrons thus must migrate large distances to the surface where they can be collected in the image sensor pixels. During the process of migration, electrons can spread laterally leading to loss of MTF, or they can be lost completely due to recombination. To minimize these problems the large depletion depth is formed that provides vertical fields, which drive the generated electrons quickly to the surface pixels before they have a chance to spread or recombine.
The use of high resistivity substrates is not without problems, however. It is very difficult to prepare them with sufficient quality, purity, and the required low doping level. The poor quality then causes unpleasant image defects and non-uniformities as well as high dark current. Some improvements can be obtained by device cooling, but this solution is expensive and many times unacceptable due to the large package size and increased power consumption. Tsoi et al. has described the high resistivity approach in: xe2x80x9cA Deep-Depletion CCD Imager for Soft X-Ray, Visible, and Near-Infrared Sensingxe2x80x9d published in IEEE Transactions on Electron Devices vol. ED-32, No. 8, August 85, pp. 1525-1530. More recently, the high resistivity substrates were discussed by Burke et al. in: xe2x80x9cAn Abuttable CCD Imager for Visible and X-Ray Focal Plane Arraysxe2x80x9d published in the same Journal vol. ED-38, No. 5, May 91, pp. 1069-1076. However, the problems related to the high resistivity issues still remain unsolved.
It is the purpose of this invention to overcome the above-described limitations, and to achieve both high QE and high MTF in the NIR sensitive image sensors without using high resistivity substrates. The prior art does not show how to substantially increase the depletion depth of image sensors by applying a large negative bias to the substrate that has standard doping. The prior art does not teach how to design image sensor arrays and their peripheral devices on the substantially standard substrate materials that are biased with large negative biases.
The invention relates to the process for fabricating of image sensor arrays that use a large negative substrate bias, to the design of the image sensor arrays themselves, and their peripheral devices such as output diodes, transistors, and buffer amplifiers, which can correctly function on negatively biased substrates.
The present invention provides process modifications that can be used to fabricate image sensors with an enhanced QE and MTF in the NIR spectral region and that are fabricated on the standard substrates. The CCD image device of the invention provides practical high performance image sensor designs of various architectures and with various peripheral devices that have high QE and high MTF in the NIR spectral region.
The improvement to CCD and CMOS image devices achieved by replacing the p+ type doped layer, typically present under the thick field oxide in the inactive regions of the sensor, with an n+ type doped layer. The n+ type layer, which is biased at the Vdd potential, surrounds the entire image sensor array as a guard ring, and is separated from the CCD or CMOS array pixels by a suitable potential barrier. The potential barrier prevents collected charge from escaping into the n+ layer regions.
Additional embodiments include output diode and MOS transistor designs that use field plates for creating potential barriers that separate these devices from the n+ type doped field regions.
In an alternative embodiment an n type region is formed under the p+ type doped inactive regions adjacent to the array and under the channel stop regions of the array. The n type doped region is connected to the n+ type doped guard ring that surrounds the p+ type regions and to the n type buried channel region of the array. The p+ type region is biased at the ground potential, while the guard ring is biased at Vdd potential. Advantage of this arrangement is that pinned photo diodes and Virtual Phase electrodes can be used in the pixel array.
An advantage of the above-described arrangements is that a large negative bias can be supplied to the substrate without substantially affecting the function of the image sensor array and its peripheral devices. The resulting large depletion depth created in the standard concentration epi-layer minimizes problems associated with high resistivity doping while achieving high QE and high MTF.